The present invention relates to semiconductor heterojunction bipolar transistors, and more particularly to a method of fabricating a SiGe heterojunction bipolar transistor in which the SiGe bipolar yield is substantially improved by suppressing dislocations that cause collector-emitter (CE) leakage or shorts, or collector-base (CB) leakage or shorts.
Significant growth in both high-frequency wired and wireless markets has introduced new opportunities where compound semiconductors such as SiGe have unique advantages over bulk complementary metal oxide semiconductor (CMOS) technology. With the rapid advancement of epitaxial-layer pseudomorphic SiGe deposition processes, epitaxial-base SiGe heterojunction bipolar transistors have been integrated with mainstream CMOS development for wide market acceptance, providing the advantages of SiGe technology for analog and RF circuitry while maintaining the full utilization of the advanced CMOS technology base for digital logic circuitry.
It is well documented that excess interstitials created by implant damage cause the formation of dislocations in the collector and emitter regions of bipolar devices. When the dislocations extend between the collector and emitter regions, bipolar pipe shorts, i.e., collector-emitter shorts, may occur. In such a context, SiGe bipolar yield can be reduced by as much as 20 to 50% for dislocations originating in the collector region.
The incorporation of C, carbon, into SiGe heterojunction devices has been carried out in the prior art to prevent the out-diffusion of boron into the base region. For example, it is known that the transient enhanced diffusion of boron is strongly suppressed in carbon-rich silicon layers; See, for example, H. J. Osten, et al., xe2x80x9cCarbon Doped SiGe Heterojunction Bipolar Transistors for High Frequency Applicationsxe2x80x9d, IEEEBTCM 7.1, 109. Boron diffusion in silicon occurs via an interstitial mechanism and is proportional to the concentration of silicon self-interstitials. Diffusion of carbon out of the carbon-rich regions causes an undersaturation of silicon self-interstitials. As a result, the diffusion of boron in these regions will be suppressed. Despite being capable of suppressing the diffusion of boron, prior art methods that incorporate C into the SiGe heterojunction bipolar structure do not prevent bipolar pipe shorts from occurring. Thus, prior art methods do not improve the SiGe bipolar yield.
In view of the SiGe bipolar yield problem mentioned above, there is a continued need for providing a new and improved method for improving SiGe heterojunction bipolar yield due to dislocations originating in the pedestal and collector regions of the device.
One object of the present invention is to provide a method of fabricating a SiGe heterojunction bipolar transistor wherein improved SiGe bipolar yield is achieved.
Another object of the present invention is to provide a method of fabricating a SiGe heterojunction bipolar transistor in which the amount of dislocations present in the device is substantially reduced thereby avoiding pipe shorts.
A further object of the present invention is to provide a method of fabricating a SiGe heterojunction bipolar transistor using processing steps that are compatible with existing bipolar and CMOS processing steps.
These and other objects and advantages are achieved in the present invention by implanting carbon, C, into certain predetermined regions of the SiGe bipolar transistor. Specifically, applicants have determined that by incorporating C (via implantation only) into the sub-collector, the collector, the extrinsic base and the collector-base junction region of a bipolar device, separately or in any combination, improved SiGe bipolar yield can be obtained. The carbon implant(s) may be carried out by blanket or masked implant techniques well known to those skilled in the art.
The greatest enhancement and most preferred embodiment of the present invention is obtained when all the C implants, as defined hereinabove, are employed. The improved SiGe bipolar yield obtained by the present invention is a significant advancement in this art since it results in a device having substantially less pipe shorts than heretofore possible with prior art SiGe heterojunction bipolar devices.
Broadly speaking, the present invention includes a method for improving the SiGe bipolar yield which comprises the steps of:
(a) providing a structure which includes at least a bipolar device region, said bipolar device region comprising at least a collector region formed over a sub-collector region, and a SiGe layer formed over said collector and subcollector regions, said SiGe layer comprising at least an intrinsic base region and a collector-base junction region, wherein said intrinsic base region is abutted by extrinsic base regions; and
(b) implanting C into at least one region of said structure selected from said collector, said sub-collector, said extrinsic base regions and said collector-base junction region.
In one embodiment of the present invention, the SiGe layer is grown utilizing a non-selective epi process. In this embodiment, the SiGe layer would include extrinsic base regions abutting the intrinsic base region. In other embodiments, the SiGe layer is formed without extrinsic base regions. In that embodiment, the extrinsic base regions, which may or may not include germanium, are formed separately from the SiGe layer.
In a preferred embodiment of the present invention, the method of the present invention comprises the steps of:
(a) providing a structure which includes at least a bipolar device region, said bipolar device region comprising at least a collector region formed over a sub-collector region;
(b) implanting C into said collector and said sub-collector regions;
(c) forming a SiGe layer on said bipolar device region, said SiGe layer comprising at least an intrinsic base region and a collector-base junction region, wherein said intrinsic base region is abutted by extrinsic base regions;
(d) implanting C into said extrinsic base regions;
(e) forming an insulator layer on said SiGe layer;
(f) providing an emitter opening in said insulator layer so as to expose a portion of said intrinsic base region and implanting C through said emitter opening and through the exposed portion of said intrinsic base region into the collector-base junction region; and
(g) forming an emitter polysilicon region on said insulator layer, including in said emitter opening.
A further aspect of the present invention relates to a SiGe heterojunction bipolar transistor that has improved SiGe bipolar yield. Specifically, the inventive SiGe heterojunction bipolar transistor comprises:
a semiconductor substrate of a first conductivity type including at least a sub-collector region and a collector region;
a SiGe base layer formed on said substrate, said SiGe base layer comprising at least collector-base junction region formed over the collector region and an intrinsic base region, wherein said intrinsic base region is abutted by extrinsic base regions; and
an emitter region formed on a portion of said intrinsic base region, said emitter region comprising at least an emitter polysilicon region, wherein at least one region of said structure selected from said collector, said sub-collector, said extrinsic base regions and said collector-base junction region includes a C implant.
In another preferred embodiment of the present invention, the bipolar transistor comprises:
a semiconductor substrate of a first conductivity type including at least a sub-collector region and a collector region which are both doped with implanted C;
a SiGe base layer formed on said substrate, said SiGe base layer comprising at least a collector-base junction region formed over the collector region, an intrinsic base region and extrinsic base regions abutting said intrinsic base region, wherein said collector-base junction region and said extrinsic base regions are doped with implanted C; and
an emitter region formed on a portion of said intrinsic base region, said emitter region comprising at least an emitter polysilicon region.
Note that each of the C-doped regions is formed by implanting C into the specific region by utilizing the method of the present invention.